1. Field of the Invention
The present invention relates to a portable infrared (IR) video camera configured to render and display a video image of an invisible gas plume such as a gas or vapor leaking into the atmosphere from a container or conduit. More specifically the invention is a portable video camera configured to render and display an image of compounds that absorb infrared radiation in the wavelength range of 10.3-10.8 μm.
2. Description of the Related Art
“Leak detection and repair” (LDAR) is a common problem in commercial applications where various substances are processed, stored, distributed, and utilized. In the petrochemical industry, leak detection devices include sniffers, scanners and passive imaging devices configured to identify a petrochemical leak by sensing the absorption of infrared radiation by the leaking compound at one or more predetermined infrared absorption bandwidths. In particular, methane (CH4), has strong infrared absorption bands approximately centered at the non-visible wavelengths 1.33 μm, 1.67 μm, 3.3 μm and 7.6 μm, and it is know to construct leak detecting devices to determine if methane is present in a gas sample by determining if the gas sample absorbs radiation at one or more of the methane absorption wavelengths. Similarly, other compounds can be detected by leak detection devices tuned to determine if other compounds are present in a gas sample by determining if the gas sample absorbs radiation at one or more absorption bands associated with the other compounds.
One example of a sniffer device is disclosed in U.S. Pat. No. 7,022,993 to Williams II et al. The sniffer device draws a gas sample into a chamber through a probe, transmits an infrared radiation beam through the gas sample to a photo detector, and a photo detector response signal is used to determine if the gas sample is absorbing infrared radiation at one or more predetermined absorption bands. One problem with using a sniffer device to detect gas leaks is that the probe must take in a gas sample directly from the leak plume in order to detect the leak. Accordingly, in a large facility or along miles of distribution conduits, leak detection by using a sniffer device is often inefficient and unreliable because leaks can be missed. Moreover, a user must be able to place the probe in the leak plume and this may not always be practical.
One example of a scanner device, called a laser methane detector, is disclosed in U.S. Pat. No. 7,075,653 to Rutherford. The laser methane detector scans a survey area with a tunable IR laser diode emitter and analyzes IR radiation reflected back from the survey area to a photo detector. If the presence of a methane plume is detected in the survey area, the laser methane detector alerts an operator by sounding an audible alarm. The tunable IR laser diode emitter is tuned over a range of wavelengths that includes in-band wavelengths, (approximately 1.67 μm), that are absorbed methane, and out-of-band wavelengths that are not absorbed by methane and to use the photo detector response to determine if methane is present. The laser methane detector provides an advantage over a sniffer because the laser methane detector can detect a methane gas plume from a remote distance. However, one problem with the laser methane detector disclosed by Rutherford is that the tunable IR laser emitter is limited to emitting over a wavelength range of about 1.2-2.5 μm. Accordingly, the laser methane detector is only usable to detect compounds with a strong absorption band within the wavelength range of about 1.2-2.5 μm.
One example of a passive imaging device configured to detect the presence of methane and other hydrocarbon gas plumes is a video thermography camera disclosed in U.S. patent application Ser. No. 11/298,862, by Furry, which was published as US2006/0091310A1 and as WO2005001409. A second example of video thermography camera configured to detect the presence of methane and other hydrocarbon gas plumes is commercially available from FLIR SYSTEM Inc. of Wilsonville, Oreg. and North Billerica, Mass., USA; sold under the trade name ThermaCam® GasFindIR™.
Both example thermography camera examples include a lens positioned to form an image of a survey scene that may contain an infrared absorbing gas plume. The image of the survey scene is focused onto a focal plane array and an optical band pass filter is positioned between the lens the focal plane array to limit the spectral bandwidth of the image of the survey scene to a desired wavelength range. The desired wavelength range corresponds with an absorption band of a compound that it is desired to detect in the image of the survey scene.
The example thermography cameras each include signal processing elements configured to render and display a video image of the limited spectral bandwidth survey scene such that a leak plume that contains a compound having an absorption band corresponding with or at least partially overlapping the limited spectral bandwidth is rendered visible in the displayed video image. A user viewing the displayed video image can then study the leak plume to determine its source or otherwise study its dynamics in real time.
The example thermography cameras each include a cryocooler refrigeration device, or container of liquid nitrogen, for cooling the focal plane array and the optical filter, (cold filter), to 77 to 100° K., during operation, in order to reduce thermal energy from radiating from the focal plane array and the optical filter in order to reduce signal noise and increase contract of the leak plume with respect to background elements of the survey scene image.
The example thermography cameras each include a focal plane array that comprises Indium Antimonide, (InSb) IR photo sensor elements. InSb photo sensor elements have a usable responsivity over the approximate spectral range of 1-5.5 μm, but are more practically limited to a usable range of 3.0-5.0 μm. Accordingly, the example thermography cameras are practically limited to detecting leak plume containing compounds that have absorption bands in the spectral range of 3.0-5.0 μm. While that range is suitable for detecting methane and other hydrocarbon compound leaks, there is a need for a thermographic leak detector that operates to detect compounds having absorption bands above 5.0 μm.
Furry suggests using a thermographic camera equipped with an optical filter tuned to wavelengths above 5.5 μm to detect ethylene, (approximately 10.5 μm) propylene, (approximately 10.9 μm), butadiene, (approximately 11.1 μm) and sulfur hexafluoride (SF6), (approximately 10.5 μm), however, Furry is completely silent regarding what focal plane array technology would be suitable for such a thermographic video camera.
Another problem with conventional thermographic leak detection systems is that InSb focal plane arrays have a broad spectral responsivity, e.g. 2 μm, as compared to typical absorption bands, which may have a spectral bandwidth of 0.1-0.3 μm. The problem is that the extra spectral responsivity range of the InSb focal plane arrays contributes dark current signal noise that ultimately reduces the contrast of the leak plume as compared to the background of a video survey image. Accordingly, it is desirable to use a photo sensor that has a spectral responsivity range that is spectrally tuned to the absorption bandwidth of the compound to be imaged in order to increase image contrast.
Additionally, in the other industries, notably electrical power distribution, there is a need for a thermography camera for detecting leaks of the industrial gas sulfur hexafluoride (SF6). SF6 is commonly used as an electrical insulator and has a strong absorption band at approximately 10.57 μm. Conventional thermography cameras do not have a focal plane array capable of forming an image of a survey scene over a wavelength range that includes 10.57 μm.